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/* -*- Mode: C; c-basic-offset:4 ; indent-tabs-mode:nil -*- */
/*
* Copyright (c) 2004-2005 The Trustees of Indiana University and Indiana
* University Research and Technology
* Corporation. All rights reserved.
* Copyright (c) 2004-2007 The University of Tennessee and The University
* of Tennessee Research Foundation. All rights
* reserved.
* Copyright (c) 2004-2005 High Performance Computing Center Stuttgart,
* University of Stuttgart. All rights reserved.
* Copyright (c) 2004-2005 The Regents of the University of California.
* All rights reserved.
* Copyright (c) 2007 Voltaire All rights reserved.
* Copyright (c) 2010 IBM Corporation. All rights reserved.
* Copyright (c) 2014-2018 Los Alamos National Security, LLC. All rights
* reseved.
* $COPYRIGHT$
*
* Additional copyrights may follow
*
* $HEADER$
*/
#ifndef OPAL_FIFO_H_HAS_BEEN_INCLUDED
#define OPAL_FIFO_H_HAS_BEEN_INCLUDED
#include "opal_config.h"
#include "opal/class/opal_lifo.h"
#include "opal/sys/atomic.h"
#include "opal/threads/mutex.h"
BEGIN_C_DECLS
/* Atomic First In First Out lists. If we are in a multi-threaded environment then the
* atomicity is insured via the compare-and-swap operation, if not we simply do a read
* and/or a write.
*
* There is a trick. The ghost element at the end of the list. This ghost element has
* the next pointer pointing to itself, therefore we cannot go past the end of the list.
* With this approach we will never have a NULL element in the list, so we never have
* to test for the NULL.
*/
struct opal_fifo_t {
opal_object_t super;
/** first element on the fifo */
volatile opal_counted_pointer_t opal_fifo_head;
/** last element on the fifo */
volatile opal_counted_pointer_t opal_fifo_tail;
/** list sentinel (always points to self) */
opal_list_item_t opal_fifo_ghost;
};
typedef struct opal_fifo_t opal_fifo_t;
OPAL_DECLSPEC OBJ_CLASS_DECLARATION(opal_fifo_t);
static inline opal_list_item_t *opal_fifo_head (opal_fifo_t* fifo)
{
return (opal_list_item_t *) fifo->opal_fifo_head.data.item;
}
static inline opal_list_item_t *opal_fifo_tail (opal_fifo_t* fifo)
{
return (opal_list_item_t *) fifo->opal_fifo_tail.data.item;
}
/* The ghost pointer will never change. The head will change via an atomic
* compare-and-swap. On most architectures the reading of a pointer is an
* atomic operation so we don't have to protect it.
*/
static inline bool opal_fifo_is_empty( opal_fifo_t* fifo )
{
return opal_fifo_head (fifo) == &fifo->opal_fifo_ghost;
}
#if OPAL_HAVE_ATOMIC_COMPARE_EXCHANGE_128
/* Add one element to the FIFO. We will return the last head of the list
* to allow the upper level to detect if this element is the first one in the
* list (if the list was empty before this operation).
*/
static inline opal_list_item_t *opal_fifo_push_atomic (opal_fifo_t *fifo,
opal_list_item_t *item)
{
opal_counted_pointer_t tail = {.value = fifo->opal_fifo_tail.value};
const opal_list_item_t * const ghost = &fifo->opal_fifo_ghost;
item->opal_list_next = (opal_list_item_t *) ghost;
opal_atomic_wmb ();
do {
if (opal_update_counted_pointer (&fifo->opal_fifo_tail, &tail, item)) {
break;
}
} while (1);
opal_atomic_wmb ();
if (ghost == tail.data.item) {
/* update the head */
opal_counted_pointer_t head = {.value = fifo->opal_fifo_head.value};
opal_update_counted_pointer (&fifo->opal_fifo_head, &head, item);
} else {
/* update previous item */
tail.data.item->opal_list_next = item;
}
return (opal_list_item_t *) tail.data.item;
}
/* Retrieve one element from the FIFO. If we reach the ghost element then the FIFO
* is empty so we return NULL.
*/
static inline opal_list_item_t *opal_fifo_pop_atomic (opal_fifo_t *fifo)
{
opal_list_item_t *item, *next, *ghost = &fifo->opal_fifo_ghost;
opal_counted_pointer_t head, tail;
opal_read_counted_pointer (&fifo->opal_fifo_head, &head);
do {
tail.value = fifo->opal_fifo_tail.value;
opal_atomic_rmb ();
item = (opal_list_item_t *) head.data.item;
next = (opal_list_item_t *) item->opal_list_next;
if (ghost == tail.data.item && ghost == item) {
return NULL;
}
/* the head or next pointer are in an inconsistent state. keep looping. */
if (tail.data.item != item && ghost != tail.data.item && ghost == next) {
opal_read_counted_pointer (&fifo->opal_fifo_head, &head);
continue;
}
/* try popping the head */
if (opal_update_counted_pointer (&fifo->opal_fifo_head, &head, next)) {
break;
}
} while (1);
opal_atomic_wmb ();
/* check for tail and head consistency */
if (ghost == next) {
/* the head was just set to &fifo->opal_fifo_ghost. try to update the tail as well */
if (!opal_update_counted_pointer (&fifo->opal_fifo_tail, &tail, ghost)) {
/* tail was changed by a push operation. wait for the item's next pointer to be se then
* update the head */
/* wait for next pointer to be updated by push */
while (ghost == item->opal_list_next) {
opal_atomic_rmb ();
}
opal_atomic_rmb ();
/* update the head with the real next value. note that no other thread
* will be attempting to update the head until after it has been updated
* with the next pointer. push will not see an empty list and other pop
* operations will loop until the head is consistent. */
fifo->opal_fifo_head.data.item = (opal_list_item_t *) item->opal_list_next;
opal_atomic_wmb ();
}
}
item->opal_list_next = NULL;
return item;
}
#else
/* When compare-and-set 128 is not available we avoid the ABA problem by
* using a spin-lock on the head (using the head counter). Otherwise
* the algorithm is identical to the compare-and-set 128 version. */
static inline opal_list_item_t *opal_fifo_push_atomic (opal_fifo_t *fifo,
opal_list_item_t *item)
{
const opal_list_item_t * const ghost = &fifo->opal_fifo_ghost;
opal_list_item_t *tail_item;
item->opal_list_next = (opal_list_item_t *) ghost;
opal_atomic_wmb ();
/* try to get the tail */
tail_item = opal_atomic_swap_ptr (&fifo->opal_fifo_tail.data.item, item);
opal_atomic_wmb ();
if (ghost == tail_item) {
/* update the head */
fifo->opal_fifo_head.data.item = item;
} else {
/* update previous item */
tail_item->opal_list_next = item;
}
opal_atomic_wmb ();
return (opal_list_item_t *) tail_item;
}
/* Retrieve one element from the FIFO. If we reach the ghost element then the FIFO
* is empty so we return NULL.
*/
static inline opal_list_item_t *opal_fifo_pop_atomic (opal_fifo_t *fifo)
{
const opal_list_item_t * const ghost = &fifo->opal_fifo_ghost;
#if OPAL_HAVE_ATOMIC_LLSC_PTR
register opal_list_item_t *item, *next;
int attempt = 0, ret = 0;
/* use load-linked store-conditional to avoid ABA issues */
do {
if (++attempt == 5) {
/* deliberatly suspend this thread to allow other threads to run. this should
* only occur during periods of contention on the lifo. */
_opal_lifo_release_cpu ();
attempt = 0;
}
opal_atomic_ll_ptr(&fifo->opal_fifo_head.data.item, item);
if (ghost == item) {
if (ghost == fifo->opal_fifo_tail.data.item) {
return NULL;
}
/* fifo does not appear empty. wait for the fifo to be made
* consistent by conflicting thread. */
continue;
}
next = (opal_list_item_t *) item->opal_list_next;
opal_atomic_sc_ptr(&fifo->opal_fifo_head.data.item, next, ret);
} while (!ret);
#else
opal_list_item_t *item, *next;
/* protect against ABA issues by "locking" the head */
do {
if (!opal_atomic_swap_32 ((volatile int32_t *) &fifo->opal_fifo_head.data.counter, 1)) {
break;
}
opal_atomic_wmb ();
} while (1);
opal_atomic_wmb();
item = opal_fifo_head (fifo);
if (ghost == item) {
fifo->opal_fifo_head.data.counter = 0;
return NULL;
}
next = (opal_list_item_t *) item->opal_list_next;
fifo->opal_fifo_head.data.item = next;
#endif
if (ghost == next) {
void *tmp = item;
if (!opal_atomic_compare_exchange_strong_ptr (&fifo->opal_fifo_tail.data.item, &tmp, (void *) ghost)) {
do {
opal_atomic_rmb ();
} while (ghost == item->opal_list_next);
fifo->opal_fifo_head.data.item = (opal_list_item_t *) item->opal_list_next;
}
}
opal_atomic_wmb ();
/* unlock the head */
fifo->opal_fifo_head.data.counter = 0;
item->opal_list_next = NULL;
return item;
}
#endif
/* single threaded versions of push/pop */
static inline opal_list_item_t *opal_fifo_push_st (opal_fifo_t *fifo,
opal_list_item_t *item)
{
opal_list_item_t *prev = opal_fifo_tail (fifo);
item->opal_list_next = &fifo->opal_fifo_ghost;
fifo->opal_fifo_tail.data.item = item;
if (&fifo->opal_fifo_ghost == opal_fifo_head (fifo)) {
fifo->opal_fifo_head.data.item = item;
} else {
prev->opal_list_next = item;
}
return (opal_list_item_t *) item->opal_list_next;
}
static inline opal_list_item_t *opal_fifo_pop_st (opal_fifo_t *fifo)
{
opal_list_item_t *item = opal_fifo_head (fifo);
if (item == &fifo->opal_fifo_ghost) {
return NULL;
}
fifo->opal_fifo_head.data.item = (opal_list_item_t *) item->opal_list_next;
if (&fifo->opal_fifo_ghost == opal_fifo_head (fifo)) {
fifo->opal_fifo_tail.data.item = &fifo->opal_fifo_ghost;
}
item->opal_list_next = NULL;
return item;
}
/* push/pop versions conditioned off opal_using_threads() */
static inline opal_list_item_t *opal_fifo_push (opal_fifo_t *fifo,
opal_list_item_t *item)
{
if (opal_using_threads ()) {
return opal_fifo_push_atomic (fifo, item);
}
return opal_fifo_push_st (fifo, item);
}
static inline opal_list_item_t *opal_fifo_pop (opal_fifo_t *fifo)
{
if (opal_using_threads ()) {
return opal_fifo_pop_atomic (fifo);
}
return opal_fifo_pop_st (fifo);
}
END_C_DECLS
#endif /* OPAL_FIFO_H_HAS_BEEN_INCLUDED */
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